According to the widespread GSM (Global System for Mobile Communication) mobile radio standard, phase modulation and frequency modulation are used for processing digital data to be transmitted. In this case, Gaussian Minimum Shift Keying (GMSK) is normally used for digital modulation of the data to be transmitted, in which the payload signal information is not coded in the signal amplitude of the radio signal to be transmitted via a channel, but only in the phase and/or frequency.
The demand for higher transmission rates in mobile radio has lead to the introduction of modulation methods for higher data rates, such as the extension to the GSM Standard, the so-called GSM EDGE system. In this case, the signal information is represented by a combination of phase and amplitude keying. The abbreviation EDGE in this case stands for Enhanced Data Rates for GSM Evolution. EDGE allows transmission rates which are comparable to UMTS (Universal Mobile Telecommunications System).
In GSM systems which operate using GMSK modulation methods, the transmission architectures are normally based on phase locked loops which use a VCO (Voltage Controlled Oscillator) which is driven by the modulation signal, or phase locked loops with a delta-sigma modulator are used. Transmission arrangements such as these have a good noise response with a low power consumption, and do not require any additional surface acoustic wave filters at their output.
Alternatively, it is also possible to use transmission architectures with a quadrature modulator. In this case, the quadrature modulator may, for example, be in the form of a vector modulator, and may be driven by a complex-value baseband signal. In this case, direct conversion to the transmission frequency band is normally provided. However, quadrature modulators such as these have the disadvantage that the power consumption is relatively high in order to achieve acceptable noise characteristics. Furthermore, an external surface acoustic wave filter is required for bandpass filtering on the output side.
While the quadrature modulator described most recently above is suitable for processing phase-modulated and amplitude-modulated signals, such as those which occur with GSM EDGE, no amplitude information can be processed when using the power-saving architectures in which a VCO is driven with a radio-frequency signal that has already been modulated.